This invention relates to a superconducting coil protective system for protecting a superconducting coil from being destroyed upon quenching.
FIGS. 6 to 9 illustrate circuit diagrams of various superconducting coil protective systems disclosed in "Improvements in the Parallel Resistor Circuit for the Quench Protection of a Superconducting Magnet" by T. Nakano, S. Okuma and Y. Amamiya, B-102 No. 12 pp. 73-79, published from Japanese Institute of Electrical Engineering, December, 1982.
In the conventional parallel resistor circuit type protective system illustrated in FIG. 6, a cryostat CR comprises a superconducting coil L and a resistance R(t) of a normal conduction portion generated in the superconducting coil L. This resistance R(t) has a resistance that increases as the time lapses. Across the cryostat CR, a power source such as a mono-polar electric source E is connected through a power switch S, and a protective resistor R.sub.o is connected in parallel to the cryostat CR.
FIG. 7 illustrates a protective circuit in which a diode D is employed in place of the protective resistor R.sub.o and two switches S1 and S2 as well as three resistors R1, R2 and R3 are used to form a multi-stage parallel resistors.
FIG. 8 illustrates a protective circuit in which series-connected resistors Ra and Rb are connected in parallel to the cryostat CR, and a capacitor C is connected across the resistor Rb. A protective circuit illustrated in FIG. 9 further comprises a series circuit of an inductor Ls and a resistor Rs connected in parallel to the protective resistor.
All of these known protective circuits illustrated in FIGS. 6 to 9, which comprise the power switch S connected between the power source E and the superconducting coil L, have the basically same disadvantages, so that the description of the d.c. current interrupting operation will be made only in terms of the protective circuit illustrated in FIG. 6 for simplicity.
During normal operation, the power switch S of the protective circuit of FIG. 6 is closed and a very large current from the power source E flows through the superconducting coil L but substantially no current flows through the protective resistor R.sub.D because it has a large resistance.
However, upon the occurrence of quenching in the superconducting coil L, in order to quickly remove stored energy within the superconducting coil L, as soon as the occurrence of the quenching in the superconducting coil L is detected, the voltage of the power source E is decreased and at the same time the power switch S is opened. Then a high voltage Vc which generates across the power switch S is applied to the protective resistor R.sub.D, whereupon an electric current which has been flowing through the superconducting coil L initiates to flow as indicated by an arrow i.sub.D. Then, the magnetic energy stored in the superconducting coil L is converted into heat at the protective resistor R.sub.o to be dissipated to the exterior of the cryostat CR, whereby the superconducting coil L can be protected.
With the conventional protective system as above discussed, the power switch S must carry an extremely massive current, which also flows through the superconducting coil L, during normal operating condition, and also the power switch S must interrupt this massive current at a high voltage upon the occurrence of the quenching in the superconducting coil L. However, it sometimes happens that the power switch S fails to interrupt the current when the arc voltage of the arc plasma generated across the contacts of the power switch S is low and does not reach the interrupting voltage. If the current interruption is failed, the power switch S and the superconducting coil L are destroyed.
In the circuit illustrated in FIG. 6, since the current must be interrupted by the power switch S, the power switch S must have a d.c. current interruption capability which can break a current ic at a voltage (RD.times.ic). Also, because of an arc plasma generated between the switch contacts of the power switch S, the current interruption sometimes fails and it is difficult to reliably carry out the interruption.
FIG. 10 illustrates a further example of a conventional superconducting coil protective system in which a reversible polarity power source PS is connected across the superconducting coil L through a disconnector DS. The system also includes a protective resistor RD connected in parallel to the superconducting coil L. A closing switch S3 is connected in series to the protective resistor RD and a fuse F is connected in parallel to the the resistor RD. The normal conduction portion of the superconducting coil L is not illustrated in FIG. 10.
During normal operation, the disconnector DS is closed so that a predetermined current i can flow into the superconducting coil l in the direction of the arrow from the reversible polarity power source PS to energize the superconducting coil L.
Upon the occurrence of quenching in the superconducting coil L, the quenching is detected and the closing switch S3 is immediately closed and the voltage across the reversible polarity power source is reversed or made zero. Then, the current from the power source PS rapidly decreases and the current in the superconducting coil L is commutated into the fuse F as illustrated by the arrow ic, so that the current from the power source PS eventually reduces to zero, upon which the disconnector DS can be opened. Thereafter, the fuse F melts because of the commutated current. Therefore, the commutated current which has been flowing through the fuse is interrupted and again commutated to the protective resistor RD where the energy in the superconducting coil L is dissipated.
In this arrangement, it is not possible to control the relationship between the time points of the closure of the closing switch S3 and the current commutation into the fuse F, so that the arrangement is disadvantageous in that a large-sized fuse must be used as the fuse F because the current interruption fails if the fuse F melts before the current commutation completes, and that it is difficult to simultaneously interrupt the current in the system having a plurality of d.c. interrupting units.